1. Field of the Invention
The invention relates generally to metallic ink compositions for use in capillary-action markers. More particularly, the invention relates to a metallic ink composition comprised of a polar solvent, an encapsulated metallic pigment, and a resin component.
2. Brief Description of Related Technology
Writing instruments capable of making written markings containing metallic pigments are known. For example, valve-action markers capable of making written markings containing metallic pigments have been developed. Typically, such valve-action markers utilize a spring-loaded nib, which opens a valve to an ink reservoir when depressed (e.g., against a writing surface), thereby allowing the ink to flow from the ink reservoir to the nib.
Valve-action markers require more parts than conventional capillary-action markers, and consequently their manufacture is more complicated and costly. Additionally, valve-action markers are problematic in that the metallic pigments tend to settle to the bottom of the ink reservoir when the valve-action markers are not in use. Therefore, the user often has to violently shake the marker prior to using same in order to effect distribution of the metallic pigments throughout the ink composition so as to ensure that the ink composition delivered to the marker nib contains sufficient amounts of metallic pigment to produce the desired visual effect. Moreover, the user typically has no means to verify that the metallic pigment has been adequately distributed throughout the ink composition without writing with the marker because the marker barrel is opaque. The user must also subsequently depress the nib against a writing surface with sufficient force to open the valve to the ink reservoir and allow delivery of the ink composition to the nib as described above. Such forces can cause the nib to excessively wear such that any written markings produced using the same frequently become indistinct over time. In view of the foregoing, other means of delivering metallic ink compositions are desired.
Conventional capillary-action markers typically contain a fibrous ink reservoir and a nib in fluid communication therewith. Such markers typically include an ink composition having a low viscosity because the adhesive forces (between the ink composition and the channel walls of the reservoir and/or nib) must exceed the cohesive forces of the ink composition to permit movement of the composition by capillary-action. Incorporating metallic pigments (e.g., including various metallic pigments such as aluminum and bronze flakes) into the low viscosity ink compositions used in capillary-action markers is often challenging because such metallic pigments tend to settle out of the ink compositions as set forth above. Even when the metallic pigments are adequately suspended in the ink compositions, the marker's fibers frequently undesirably “filter” the metallic pigments and become clogged over time. Accordingly, over time, the marker can be rendered incapable of making written markings containing metallic pigments.
U.S. Pat. No. 6,120,590 to Miyamoto discloses a ball point pen containing a water-based, thixotropic gel ink having metallic lustrous color. Such thixotropic gel inks, however, are too viscous to be successfully adapted for use in many writing instruments, including conventional capillary-action markers.
Capillary action markers including inks containing metallic pigments are, however, known in the art. U.S. Pat. No. 7,135,507 to Sexton discloses a capillary action marker containing an ink including an aqueous dispersion of a metallic pigment and a specific hydantoin-formaldehyde co-polymer film-forming resin, optionally together with one or more of a color agent, a pH-adjusting agent, anti-settling agent, or a preservative. Additionally, U.S. Pat. No. 7,297,729 to Sexton discloses a capillary action marker containing an ink including a nitro solvent, a film-forming resin, a pigment or dye, and optionally, a substrate wetting agent. Still further, U.S. Pat. No. 6,402,412 to Sukhna discloses an aqueous ink for a capillary action marker including a permanent water-based binder, a colored pigmented permanent water-based dispersion of sub-micron particle size, an aluminum dispersion of specific particle size, a humectant, a surfactant, an anti-settling additive, a preservative and a pH adjuster. Too frequently, however, capillary action markers containing known ink formulations do not deliver a substantially homogenous ink composition for a reasonable amount of time (much less so over the lifetime of the product) and tend to clog. Such problems are believed in part to be attributable to the density, size, morphology, and surface treatment of the metallic pigments often used in writing compositions.
Further, conventional metallic marker ink compositions often include resins that have substantially hydrophobic domains. While such resins have generally been necessary to facilitate metallic pigment delivery from the marker capillary action ink delivery system, these resins (due at least in part to their substantially hydrophobic domains) do not readily dissolve in polar (in particular, alcohol) solvents and therefore significantly non-polar solvents must be used. Unfortunately, the aggressive non-polar solvents used in these compositions have been found to swell and/or attack marker components made from inexpensive thermoplastics such as polypropylene and polyethylene. As a result, more expensive thermoplastic components manufactured from engineering thermoplastics such as nylon need to be used to obtain capillary action markers containing these conventional metallic ink compositions.